Natural or synthetic fibers used as textiles or compounded extenders are often coated with additives. This coating has the aim of modifying the surface properties of the fiber or granting it a specific functionality. In certain cases, the term “bonding” can be used. For example, the so-called “textile” bonding applied to filaments output from a spinneret consists of depositing a bonding agent ensuring cohesion of the filaments with each other, reducing abrasion and facilitating subsequent handling (weaving) and preventing the formation of electrostatic charges. There are other cases wherein a fiber must be covered with a specific compound. For example, it is possible to dye a fiber simply by coating it with dyeing agents. An initially insulating textile fiber can be made conductive by coating it with conductive polymers. It is possible to perfume a garment of clothing by coating its fibers with capsules containing a perfume. These are only a few examples among a multitude of industrially developed and commercially available cases.
Conventional fiber coating results in uniform, symmetric coating of its surface.
It is preferable, however, in certain cases to add an additive to a fiber in a different manner, which is to say inside the fiber and not on the surface. These different conditions make it possible to improve the properties of the fiber, allowing it to be used for new functions.
Carbon nanotubes have a structure and electronic and mechanical properties which make them very promising materials for many applications: composites, electromechanical actuators, cables, resisting wire, chemical detectors, hydrogen storage, electron-emitting displays, energy converters, electronic components, electrodes, batteries, catalysis media or the like.
Multiple methods exist for manufacturing carbon nanotube fibers.
In particular, carbon nanotubes as well as other types of particles can be arranged in the shape of ribbons or fibers by a patented spinning method (FR 2 805 179). That method consists of homogeneously dispersing the nanotubes in a liquid environment. The dispersion can be carried out in water using surface-active agents which are adsorbed at the interface of the nanotubes. It can also be obtained from functionalized nanotubes, without using any dispersants. Once dispersed, the nanotubes are re-condensed in the form of a ribbon or a pre-fiber by injecting the dispersion into another liquid causing the nanotubes to coagulate. This other liquid can be a solution of polymers. The flows used are optimized to promote alignment of the nanotubes in the pre-fiber or the ribbon. Moreover, the flow speeds and rates also make it possible to control the cross-section of the pre-fibers or ribbons. The pre-fiber is then dried, resulting in a fiber containing a considerable fraction of nanotubes. The ribbons, pre-fibers or final fibers can be treated by stretching in a wet method to improve the direction of the nanotubes. These reshaping methods are described in FR 01/10611. That patent shows how dynamically or statically stretching the fiber in solvents with a higher or lower affinity for the coagulating polymer makes it possible to improve the structure and the physical properties of the fibers.
The pre-fibers, ribbons or fibers can also be washed by rinsing which makes it possible partially or entirely to desorb certain adsorbed species (in particular, coagulating or surface-active polymers).
The properties of these fibers, as those of any other fibers, depend in a critical manner on the nature and arrangement of their components.
It can be desirable, in particular, to add an extra component to the fiber to improve its properties or grant it a particular function (optical, bio-activity, electrical or thermal properties, oxidation-reduction properties, catalytic properties, bactericidal properties, mechanical properties or the like). Currently, these improvements or functions can only be controlled by the nature of the molecules used when synthesizing the fibers. This is a serious limitation since it is not obvious to combine a given function with the conditions required to manufacture the fiber. It is therefore ideal to add these additives when synthesizing the fibers, since the additives can then be located inside the fibers. They are better protected this way. They are in direct contact with all the nanotubes, directly affecting the properties of the fiber.
However, the addition of molecular additives during synthesis greatly complicates the spinning process and can even make it impossible. For example, a molecule with a specific function can turn out to be harmful for the coagulation of the nanotubes or even for the stability of the initial dispersion. Likewise, the molecules designed to be added might not be compatible with the spinning process, if they are simply not soluble in the solvents used. For one or more of these reasons, the molecules are not added when manufacturing the fibers. They must be deposited at the end on the fibers once manufactured. However, post-synthesis coating, which is standard in spinning and textile technologies, also has limitations insofar as it does not allow a specific compound to be placed inside the fibers. The additives remain localized on the surface, which restricts their action and their effect on the fiber.